1. Field of the Invention
This invention relates to a method and apparatus for reproducing speech signals in which an input speech signal is divided into plural frames as units and encoded to find encoding parameters based on which at least sine waves are synthesized for reproducing the speech signal. The invention also relates to a method for transmitting modified encoding parameters obtained on interpolating the encoding parameters.
2. Description of the Related Art
There are currently known a variety of encoding methods for compressing signals by exploiting statistic properties of the audio signals, inclusive of speech signals and sound signals, in the time domain and in the frequency domain, and psychoacoustic characteristics of the human auditory system. These encoding methods are roughly classified into encoding on the time domain, encoding on the frequency domain and encoding by analysis/synthesis.
Meanwhile, with the high-efficiency speech encoding method by signal processing on the time axis, exemplified by code excited linear prediction (CELP), difficulties are met in speed conversion (modification) of the time axis because of rather voluminous processing operations of signals outputted from a decoder.
In addition, the above method cannot be used for e.g. pitch rate conversion because speed control is carried out in the decoded linear range.
In view of the foregoing, it is an object of the present invention to provide a method and apparatus for reproducing speech signals and a method for transmission of speech signals, in which the speed control of an arbitrary rate over a wide range can be carried out easily with high quality with the phoneme and the pitch remaining unchanged.
In one aspect, the present invention provides a method for reproducing an input speech signal based on encoding parameters obtained by splitting the input speech signal in terms of pre-set frames on the time axis and encoding the thus split input speech signal on the frame basis, comprising the steps of interpolating the encoding parameters for finding modified encoding parameters associated with desired time points and generating a modified speech signal different in rate from said input speech signal based on the modified encoding parameters. Thus the speed control at an arbitrary rate over a wide range can be performed with high signal quality easily with the phoneme and the pitch remaining unchanged.
In another aspect, the present invention provides an apparatus for reproducing a speech signal in which an input speech signal is regenerated based on encoding parameters obtained by splitting the input speech signal in terms of pre-set frames on the time axis and encoding the thus split input speech signal on the frame basis, including interpolation means for interpolating the encoding parameters for finding modified encoding parameters associated with desired time points and speech signal generating means for generating a modified speech signal different in rate from said input speech signal based on the modified encoding parameters. Thus it becomes possible to adjust the transmission bit rate. Thus the speed control at an arbitrary rate over a wide range can be performed with high signal quality easily with the phoneme and the pitch remaining unchanged.
In still another aspect, the present invention provides a method for transmitting speech signals wherein encoding parameters are found by splitting an input speech signal in terms of pre-set frames on the time axis as units and by encoding the this split input speech signal on the frame basis to find encoding parameters, the encoding parameters thus found are interpolated to find modified encoding parameters associated with a desired time point, and the modified encoding parameters are transmitted, thus enabling adjustment of the transmission bit rate.
By dividing the input speech signal in terms of pre-set frames on the time axis and encoding the frame-based signal to find encoding parameters, by interpolating the encoding parameters to find modified encoding parameters, and by synthesizing at least sine waves based upon the modified encoding parameters for reproducing speech signals, speed control becomes possible at an arbitrary rate.